Manual Elm 327 (page 14 of 94) (English)

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ELM327

ELM327DSJ Elm Electronics – Circuits for the Hobbyist

www.elmelectronics.com

AT Command Descriptions (continued)

enabled (CAF1), the formatting (PCI) bytes will be

automatically generated for you when sending, and will

be removed when receiving. This means that you can

continue to issue OBD requests (01 00, etc.) as usual,

without regard to the extra bytes that CAN diagnostics

systems require. Also, with formatting on, any extra

(unused) data bytes that are received in the frame will

be removed, and any messages with invalid PCI bytes

will be ignored. (When monitoring, however, messages

with invalid PCI bytes are all shown, with a ‘<DATA

ERROR’ message beside them).

Multi-frame responses may be returned by the

vehicle with ISO 15765 and SAE J1939. To make

these more readable, the Auto Formatting mode will

extract the total data length and print it on one line,

then show each line of data with the segment number

followed by a colon (‘:’), and then the data bytes.

You may also see the characters 'FC:' on a line (if

you are experimenting). This identifies a Flow Control

message that has been sent as part of the multi-line

message signalling. Flow Control messages are

automatically generated by the ELM327 in response to

a ‘First Frame’ reply, as long as the CFC setting is on

(it does not matter if auto formatting is on or not).

Another type of message – the RTR (or ‘Remote

Transfer Request’) – will be automatically hidden for

you when in the CAF1 mode, since they contain no

data. When auto formatting is off (CAF0), you will see

the characters 'RTR' printed when a remote transfer

request frame has been received.

Turning the CAN Automatic Formatting off (CAF0),

will cause the ELM327 to print all of the data bytes as

received. No bytes will be hidden from you, and none

will be inserted for you. Similarly, when sending data

with formatting off, you must provide all of the required

data bytes exactly as they are to be sent – the

ELM327 will not add a PCI byte for you (but it will add

some trailing 'padding' bytes to ensure that the

required eight data bytes are sent). This allows the

ELM327 to be used with protocols that have special

formatting requirements.

Note that turning the display of headers on (with

AT H1) will override some of the CAF1 formatting of

the received data, so that the received bytes will

appear much like in the CAF0 mode (ie. as received).

It is only the printing of the received data that will be

affected when both CAF1 and H1 modes are enabled,

though; when sending data, the PCI byte will still be

created for you and padding bytes will still be added.

Auto Formatting on (CAF1) is the default setting.

CEA [ turn off the CAN Extended Address ]

The CEA command is used to turn off the special

features that are set with the CEA hh command.

CEA hh [ set the CAN Extended Address to hh ]

Some (non-OBD) CAN protocols extend the

addressing fields by using the first of the eight data

bytes as a target (receiver) address. This command

allows the ELM327 to interact with those protocols.

Sending the CEA hh command causes the

ELM327 to insert the hh value as the first data byte of

all CAN messages that you send. It also adds one

more filtering step to received messages, only passing

ones that have the Tester Address in the first byte

position (in addition to requiring that ID bits match the

patterns set by AT CF and CM, or CRA). The AT CEA

hh command can be sent at any time, and changes

are effective immediately, allowing for changes of the

address ‘on-the-fly’. There is a more lengthy

discussion of extended addressing in the ‘Using CAN

Extended Addresses’ section on page 61.

The CEA mode of operation is off by default, and

once on, can be turned off at any time by sending AT

CEA, with no address. Note that the CEA setting has

no effect when J1939 formatting is on.

CF hhh [ set the CAN ID Filter to hhh ]

The CAN Filter works in conjunction with the CAN

Mask to determine what information is to be accepted

by the receiver. As each message is received, the

incoming CAN ID bits are compared to the CAN Filter

bits (when the mask bit is a ‘1’). If all of the relevant

bits match, the message will be accepted, and

processed by the ELM327, otherwise it will be

discarded. This three nibble version of the CAN Filter

command makes it a little easier to set filters with 11

bit ID CAN systems. Only the rightmost 11 bits of the

provided nibbles are used, and the most significant bit

is ignored. The data is actually stored as four bytes

internally however, with this command adding leading

zeros for the other bytes. See the CM command(s) for

more details.

CF hh hh hh hh [ set the CAN ID Filter to hhhhhhhh ]

This command allows all four bytes (actually 29

bits) of the CAN Filter to be set at once. The 3 most

significant bits will always be ignored, and may be

given any value. This command may be used to enter

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